Ligand protonation leads to highly fluorescent boronium cations

Fluorophores that respond to external stimuli, such as changes in pH, have utility in bio-imaging and sensing applications. Almost all pH-responsive fluorophores rely on complex syntheses and the use of pH-responsive functional groups that are peripheral to the fluorophore framework. In this work, pH-responsive boron-containing heterocycles based on tridentate acyl pyridylhydrazone ligands were prepared. These non-emissive heterocycles were synthesized in three steps from inexpensive, commercially available reagents without the use of chromatography or air-sensitive reagents. Treatment with acid resulted in protonation of the boron-bound methylamine donor and efficient blue photoluminescence. Experimental and computational analysis revealed that protonation changed the geometric structure of the heterocycles and prevented photoluminescence quenching associated with photoinduced electron transfer. This work demonstrates a new approach for the design of fluorophores with potential applications in biological imaging.